Digital audiovisual information is becoming widely distributed through broadcast transmission, such as digital television signals, and interactive transmission, such as the Internet. The information may be in still images, audio feeds, or video data streams. The Moving Picture Experts Group (MPEG) has promulgated a Multimedia Content Description Interface standard, commonly referred to as MPEG-7, to standardize the description of audiovisual information when it is transmitted from a system that generates the information to a system that presents the information to a user.
An MPEG-7 description defines a generic structure for particular types of multimedia content. For example, the structure for a standard movie could include scenes, shots within scenes, titles for scenes, and time, color, shape, motion, and audio for shots. The corresponding description would contain a “descriptor” component that describes one of the features of the content, such as color, shape, motion, audio, title, etc., and a “description scheme” component that describe relationships among two or more descriptors, e.g., a shot description scheme that relates the features of a shot. A description scheme can also describe the relationship among other description schemes and between description schemes and descriptors, e.g., a scene description scheme that relates the different shots in a scene and relates the title of the scene to the shots.
A Schema is designed based on descriptors and description schemes used to describe the features of multimedia content, which is encoded into an instance document using a Description Definition Language(DDL). Each descriptor entry in the schema specifies the syntax and semantics of the corresponding feature. Each description scheme entry in the schema specifies the structure and semantics of the relationships among its children components. The DDL for MPEG-7 multimedia content descriptions is based on the XML (extensible markup language) standard. The descriptors, description schemes, semantics, syntax, and structures of the content description are coded as XML elements. XML attributes can be used to specify additional information about the elements. Some of the XML elements and attributes may be optional.
An instance of a content description, such as a particular movie, is specified in an XML “instance document” that references the appropriate schema and contains a set of “descriptor values” for the required elements and attributes in the schema and for any necessary optional elements and/or attributes. An instance document is typically encoded into a binary form for transmission between the system that generated the instance document and a system that will present the multimedia content and the descriptions described in the instance document.
The more complex the multimedia content descriptions, the larger the instance document and the longer the transmission time between the generating and presentation systems. While standard compression techniques can be employed to reduce the size of the instance document when it is converted into its binary form, such techniques do not reduce the amount of data sufficiently to enable practical transmission of large instance documents over slower connections.